Yesterday as I pulled up weary tomato plants and lugged them to the compost pile, I considered my options for the empty bed. I could cover it with a winter mulch, but with a few weeks of growing season left to go, cold-hardy cover crops are a better option. From nitrogen-fixing legumes like hairy vetch and winter field peas to deeply-rooted winter grains like cereal wheat and grazing rye, winter-hardy cover crops improve the soil and green up first thing in spring - an awesome sight to a winter-weary gardener.
Experiments with Winter Legumes
Legumes including peas, beans and vetches can take nitrogen from the air and store it in their roots, so you can use cold-hardy legumes to grow your own fertilizer during the winter and early spring months. One of my favourite winter cover crops is hairy vetch (Vicia villosa). When grown before tomatoes, hairy vetch can enhance their productivity and boost tomatoes' ability to resist common diseases. Hairy vetch is also a choice cover crop to grow in areas to be planted with sweet corn, peppers or other summer crops. Planted in early fall, hairy vetch plants stay small through winter, surviving winter temperatures of -20°F (-29°C). The plants quickly grow into a thick tangle of foliage in spring. When cut off at the soil line, the foliage dries into a mulch that can be rolled up like a rug. Below ground, nitrogen nodules on rotting hairy vetch roots provide a steady supply of nutrients for the cultivated crop.
In areas where winter temperatures are not likely to drop below 0°F (-18°C), you can grow crimson clover, the most beautiful of all winter legumes. Crimson clover is an ideal cover crop for soil that won't be needed until early summer, though you have the option of turning under the plants as a green manure whenever you like. I think crimson clover's beautiful red tops are worth waiting for, and you can double your pleasure by adding blue cornflowers to the planting.
Other hardy legumes including winter field beans and winter field peas can survive winter temperatures of 10°F (-12°C), but they are such gangly plants that they are best grown in combination with upright cold-hardy grains like cereal wheat or grazing rye. In spring, when the plants are about knee-high, the entire mass can be pulled and composted, or mowed down and turned under. In Winter Grains for your Garden, I explain how to work with cereal wheat and rye by planting whole "berries" purchased at health food stores.
Using Cover Crops to Improve Drainage
In the US, quite a bit of research has gone into using daikon radish as a fall cover crop. The huge roots can penetrate compacted subsoil, and when the plants die from cold temperatures, the rotting radishes improve the soil. Indeed, any plants that grow into a lush sea of green in the fall and then die in the winter will leave behind deep pockets of organic matter. In this way, fast-growing mustard and turnips can be used as winter cover crops should you have seed to spare.
Like most gardeners, I have a fair share of dandelions in my garden, but I ignore the little seedlings I see in the autumn. When I wait until early spring to pull them out, they usually come up with a thin taproot more than a foot (30 cm) long. Humankind has yet to invent a tool that can create deep aeration holes in the soil as easily as I can pull up dandelions after a drenching spring rain. I would never plant dandelions on purpose, but they have their place among winter cover crop plants for vegetable gardens.
By Barbara Pleasant